Bottlehead Forum
Bottlehead Kits => Eros Phono => Topic started by: dave-tx on June 02, 2013, 01:51:51 PM
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Completed the Eros this afternoon. All resistance and voltage tests passed (within tolerance). Plugged it in, and was immediately blown away by the sound quality. Huge upgrade over my existing ARC SP-9 phono stage (which I actually liked a lot, prior to today). (Will post a photo in the Gallery section later)
My only question would be about gain. Both the Eros and the phono stage of the SP-9 exhibit the same "shhhh" sound at very high volumes when no music is played. The Eros (connected to the "spare" input on the SP-9), however, needs the volume turned dramatically higher to reach the same levels of volume when listening to music. Measured casually on the line-out into a recording device, my Eros is about 20+ dB lower than the SP-9 phono stage.
I'm running this with a Rega RP-3 and Rega Exact cartridge.
Is there a mod or adjustment I can do to increase gain?
Thanks,
David
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The gain of the Eros is fixed. Increasing it would make the noise louder. But give the tubes about 100 hours to run in. Often the noise goes away. My Eros is dead quiet at the levels I listen to, mid 80dB range, between tracks.
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Something doesn't sound right to me. The SP9 is not a tube based preamp. It is a hybrid and most of the gain comes from n-fets and mosfets. The Eros is all tube. The SP9 has 47dB gain phono to line. The line is 21dB, so the total gain is 68dB. The Eros is 50db. So the total gain, Eros to SP9 output through the line stage, is 70dB. That is low output moving coil range. .3-.4mV.
Your MM cartridge is very high output at 6.8 - 7.2mV. It does not need anywhere near the gain of the Eros plus the SP9. More like half, 36-40dB is more than enough. If everything is functioning properly on the Eros, you do not need more gain, you need far less. IIRC correctly, the SP9 has a jumper to reduce the line stage gain to 14dB. What is the sensitivity of your amp(s)?
If you are hearing the same "shhhh" sound from the SP9 phono and the Eros, I would look elsewhere for the source.
Sorry to sound negative. Had an SP9 for a short while, (not short enough), so please read with my bias in mind!!
Cheers,
Geary
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Just a thought, also make sure that you have not connected the Eros into the SP9 tape loop. That bypasses the line stage, hence, 20dB less!!
Cheers,
Geary
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Thanks for the tips; I've double-checked all of the connections to my preamp, and everything is as it should be.
Bear with me a bit, I haven't done much in the way of gain calculations (or even given it much thought, to be honest) for the entire signal chain before. My first thought was what you were saying - this cartridge should be giving me a LOT of output to work with...which is why my first impression was that the gain out of the Eros might be low, but as Grainger's post reminded me, that would imply that the noise floor would also be lower.
My power amp is a Bryston 3B-ST, sensitivity 1V for 120W into 8ohms, unbalanced.
I'm thinking of pulling out the o'scope and running a signal generator through the Eros to measure the actual gain I'm getting...unless that's a bad idea?
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My power amp is a Bryston 3B-ST, sensitivity 1V for 120W into 8ohms, unbalanced.
If we use 7mV for your cartridge, you need 143 gain, (1/.007), to drive the Bryston amp, 1V, to full power. That is 43dB gain, everything blasting at max volume. So, something just doesn't add up. You're positive that the Eros is not in the tape loop?
Cheers,
Geary
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Dave,
I have to ask, what do you mean by, "at high volumes with no music playing?" If you run the volume wide open with no music playing this is not a test that has meaning. It is brought to us by neurotic audio reviewers. But if you mean when listening at high volume between tracks that is more meaningful.
So if it is the latter let the tubes run in as I suggested above.
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My power amp is a Bryston 3B-ST, sensitivity 1V for 120W into 8ohms, unbalanced.
If we use 7mV for your cartridge, you need 143 gain, (1/.007), to drive the Bryston amp, 1V, to full power. That is 43dB gain, everything blasting at max volume. So, something just doesn't add up. You're positive that the Eros is not in the tape loop?
Cheers,
Geary
I'll check again this evening about the tape loop, what you're saying seems to be the only logical conclusion.
The most curious thing, to me, is looking at the levels on my recording device; normalized to where the internal SP-9 phono stage peaks at about 0dB on the recorder's meter, the Eros isn't even reaching -20dB. Monitoring the tape output should remove the line gain, and I would expect the Eros to be a few dB higher than the internal phono when measured this way. Does that seem to be the correct conclusion?
Thanks for the help,
David
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Dave,
I have to ask, what do you mean by, "at high volumes with no music playing?" If you run the volume wide open with no music playing this is not a test that has meaning. It is brought to us by neurotic audio reviewers. But if you mean when listening at high volume between tracks that is more meaningful.
So if it is the latter let the tubes run in as I suggested above.
Ah, I see what you mean; My statement was a little of both. Volume wide open with no music playing had the noise where I expected - slightly less hiss than the same wide open test of the SP-9 phono stage with no music playing.
Also, listening at what I would consider normal listening levels (not particularly high volume), between tracks. I have to crank the preamp gain up significantly higher when listening to the Eros than I do with the SP-9 phono stage to get the same level of music volume.
Thanks for the help,
David
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You will get roughly 1.5V of signal out of the Eros with 5mV in.
The AR SP-9 gives the following specs:
"Line-stage gain: 21dB to main out (CD stage 12dB lower gain), 0dB to tape out."
If we apply the full 21dB to the signal from the Eros, that will give you 16V at the output, and of course this will drop a lot if you're using the tape input or CD input.
It looks like the actual gain of the phono stage in the SP-9 is ~46dB, which is similar to the Eros, but it goes through all the gain circuitry in the SP-9, including the preamp circuit, hence the total of 67dB.
It is also very possible that all of the inputs are padded on your particular preamp, I would recommend downloading a 60Hz test tone to your phone, then sending a very low level 60Hz tone through your preamp at max volume. Keep the input level down around .1V AC (measure with your volt meter), then measure the output. You can do this for all the inputs except the phono input, as that circuitry has an EQ applied that will give you erroneous readings. I would recommend doing this regardless of any other conditions, just to understand how your preamp may be optimized.
Ultimately, two different phono preamps from two different manufacturers are very unlikely to have the exact same gain, so having to adjust the level between the two is not surprising.
-PB
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I did a little bit of testing of the preamp inputs and at quick glance found that they behaved as I would have expected; I will look more closely at them this weekend when I have time.
Although I'm still completely mystified about the lower output in general, I'm beginning to agree with Grainger that I just need to let the tubes run in and re-evaluate after that. This may be just a case of my paranoia in building my first kit, thinking there must be something I need to adjust.
I do appreciate all of the helpful suggestions!
-David
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I don't think you are going to pick up any gain. But you will see lower noise. Dan uses my Eros "Mini-Review" from a weekend with the Eros on the product page. My longer review is here after I bought and built my own:
http://www.bottlehead.com/smf/index.php/topic,2258.msg17408.html#msg17408
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I am currently away from home and can't do this myself.
If somebody could generate 1-2 minute long a 1kHz tone [0 dB point of the RIAA curve] that plays back at 5 mV through a standard consumer DAC [which is supposed to output 2 V max.] then it would be relatively easy to get a good first idea of the gain at the output of the Eros.
Computers with a soundcard seem to be more common that function generators.
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I am currently away from home and can't do this myself.
If somebody could generate 1-2 minute long a 1kHz tone [0 dB point of the RIAA curve] that plays back at 5 mV through a standard consumer DAC [which is supposed to output 2 V max.] then it would be relatively easy to get a good first idea of the gain at the output of the Eros.
Computers with a soundcard seem to be more common that function generators.
Considering that digital playback mechanisms do not have regulated AC outputs, it is impossible to generate a 5mV tone for someone else.
You can, however, download a function generator to your phone, set the frequency to 60Hz, dial down the volume while measuring the output with your meter till it hits 5mV, then turn the frequency up to 1K.
-PB
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After a couple weeks of poking and prodding, I'm still of the opinion that something's not quite right with my build. Re-measured all of the resistances and voltages, thought I'd post the ones that are off and see if any of them are critically out of range. Any opinions?
Resistances
Terminal Strip 2 and 14: 85.5k (manual lists 71k)
TS4/12: 74.8k (60k)
TS9/10: 46.6k (39k)
Socket A6/B6: 74.8k (60k)
Socket C1/C6: 46.7k (39k)
Socket C2/C7: 75k (60k)
Voltages
Kreg (1.0V) A:1.60V, B:1.58V
OB (100V) A:95.8V, B:94.8V
bA (0V) A/B: 0V
OA (170V) A:170.2V, B:166.9V
IA (225V) A:224.5V, B:220.9V
breg (100V) A:97.1V, B:95.8V
The voltages seem to be within reason, but there are a few resistances that seem significantly off. Inspection hasn't revealed any poor solder joints, but maybe I need to reflow some to make sure.
Thanks,
David
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Dave,
I kind of think that you have swapped resistors somewhere. My Eros has copious gain. My seduction replaced an ARC SP-14, big brother to the SP-9.
You might start checking the resistance across the resistors and checking against the values called for. You won't be the first builder to swap out a pair of resistors.
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Hi Grainger - Double checked all resistors, and to the best as I can tell they are all in there correctly. All transistors appear to be in the right direction as well.
My logic seems to say that if I have the same issue in both channels then it's not likely to be a poor solder joint or single bad component, because in the case of a gain issue, there would be a mismatch between channels if that were the case. Would you agree?
Looking at the schematics, the only way I can rationalize my resistance measurements is that I am not seeing the ~500Kohm resistance presented by Q2+R2 in parallel with the T2, T4, T9, A6, C1, C2 (and the corresponding B side) measurements. The only conclusion that I can draw from this is that my MJE350 transistors are backwards. I have the metal plate side facing out, as per the manual. Is this possibly incorrect for the particular transistors I received?
I'd like to get a second opinion before I desolder and reverse them. (as an aside, what is the functional consequence of having these backwards?)
EDIT: thinking about it more, I don't think these could be reversed and have any function at all. I'm still looking at this thinking that my measurements aren't taking that path into account. What does that mean, I don't know.
Thanks,
David
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Do not reverse the MJE350's, you will do a great deal of damage to the amplifier.
Please pass a 60Hz tone through your preamp and measure the output that you get from the various inputs, I stil stand by this being a very important step in the direction of diagnosis.
-PB
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David's amp is the Eros. There are no various inputs, there is just one. A 1kHz tone is much better suited for his testing than a 60Hz tone.
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This was in reference to his Counterpoint preamp, which has several different inputs padded down at different levels of attenuation.
A 60 Hz tone is measureable with a multimeter, hence my recommendation.
-PB
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Ah yes, thanks for reminding me. I haven't measured the preamp yet, but did some measurements on the Eros itself. I think these results point more to a problem in my build than an issue with the preamp.
I hooked up a few test signals, here are the results. I used a signal generator on an iPhone and measured with my scope probes. Don't know if I needed to match any particular output load(?)
All measurements peak-to-peak, 20mV input
60Hz: 1.3V out
500Hz: 0.5V
1kHz: 0.35V
10kHz: 0.30V
20kHz: 0.22V
Both channels measured approximately the same. It appears that even the 60Hz level is below nominal. Or am I fooling myself because I'm not using an inverse-RIAA source (although 1kHz should be the 0dB point on the RIAA curve, yes?)? Sorry for the clueless questions, I'm learning as I go.
Are there any good internal nodes to measure to try to track down the missing gain?
Many thanks for the continued help.
-David
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The ratio between your 60Hz measurement and your 1kHz measurement is close enough at ~ 11.4dB. That difference is the result of the RIAA filter doing its job. This tells us that your meter is reasonably accurate at 1kHz, but it's still worthwhile to find out what type of meter you used for the measurements.
That being said, you should measure way more than 0.35V at 1kHz with 20mV in. The Eros should be right around 1.5V out with 5mV in. Something inside your Eros isn't working the way it should.
20mV is a bit much on the input for your measurements [5mv would have been better], but in this case it is definitely good enough to point to the culprit.
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Good that the ratio between the frequencies seems right. I was just using an old Techtronix oscilloscope to measure both the input and output signals, probes set to 1x. So my readings involve some visual interpolation between grid marks, but they should be reasonably accurate.
I was initially using 5mV in, but 5mV/div is the smallest my scope will go, so wanted a little more resolution. I will drop it down to 5mV input for any future measurements...that's closer to what my cartridge should be putting out anyway.
Thanks,
David
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Your measurements at 10kHz and 20kHz are well below what they should be. 20kHz should be almost 20dB higher than the 1kHz value with the RIAA eq, but that issue is probably tied up with the underlying problem with your Eros. I would not worry about that now.
Your scope won't have any issues with measurement accuracy at frequencies higher than 60Hz. That's good to know.
PB will know where to look. He knows way more about the BH amps than I ever will.
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Are you using the same units for input and output measurements? Vrms?
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Are you using the same units for input and output measurements? Vrms?
Pretty sure they are the same - It's probably 15 years since I've used my oscilloscope, but I believe both scope channels are set up identically. My measurements should be peak-to-peak, since I'm "measuring" by visually looking at the waveforms.
Thanks,
David
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You should poke around socket C in the middle and also check your 6922, E88CC, 7308 tube since both of your channels seem to have the same problem. Unfortunately the wiring is rather crowded right there. Check the continuity of your ground connection(s) in that area first (ground near socket C to input ground for example).
Check the tube itself and/or replace it.
Check all resistance measurements and voltages for the connections to socket C. It gets pretty crowded in there, but you can follow the wiring to the next terminal strip. You can use page 1 of the schematic to find your way around. The socket pins are shown in the schematic: C1 and C6 should be at bit less than 170V, C2 and C7 at about 100V, C4 and C5 are the heater, C3 and C8 are the cathodes. I don't remember off the top of my head what the cathode voltage should be. There are references to the corresponding terminal strip points in the schematic. What ultimately matters is what the tube sees at its pins.
Be careful measuring directly at the socket. There isn't much room for error. You probably want to connect your probes while the amp is turned off and then turn it on to measure.
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20mV pk-pk is 7mVrms, a very good number for testing.
350mV/20mV (gain at 1kHz) is 25dB gain, well below the 48dB or so you should expect.
At 10kHz the gain is hardly down at all, it should be only 20% of that at 1kHz.
These data indicate an error in the RIAA equalization. I'm sorry but it will be a day or two before I can analyze it, hopefully either that's enough clue, or someone else will have the critical insight before then!
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In this case the scope worked just fine to document that the gain from this Eros is not what it should be. When the difference between the expected result and the actual is as great as 24dB then reading peak-to-peak voltages from a scope will do the job. Quite a few RMS reading meters can't be trusted at 20kHz and just about any scope will still display 20kHz accurately ... trade-offs everywhere.
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Sorry, no useful insight yet, beyond my previous observations. The RIAA equalization is not working correctly - there's too little bass boost, and hardly any treble cut - and the gain is more than 20dB too low at 1kHz. I see two further useful things to do:
* Experimentally, you can look with the scope at various intermediate points in the circuit. I'd suggest the EF86 grid (T19/T27), EF86 plate (A6/B6 or T4/T12), 6922 plate (T9/T10), and 6922 cathode (C3/C8). Use the scope AC coupled while you have 20mV peak to peak into the input. Your previous frequencies are fine for this.
* Mechanically, inspect the 0.030 and 0.010 capacitors to make sure they are the right values and installed in the right places, and measure the 10.5K resistor.
The resistances you measured earlier are consistent with the circuit diagram, while the ones in the manual are not. Being away from my own Eros, I can't confirm either set yet, but I thought I should mention that. Those resistances indicate to me that the resistors in the RIAA, and the plate and cathode resistors, are all the correct value, hence my emphasis on the capacitors. Your DC voltages indicate the correct DC operating points.
A couple faint possibilities also occurred to me: (These are faint because I imagine you have inspected this things way too many times already - but I list them because they are possible causes of the observed symptoms.)
* Check the integrity of the grid and plate stoppers, attached to A9, B9, C2, and C7 (220 ohm) and C1 and C6 (100 ohm)
* Check correct attachment and good solder joints and correct orientation of the 100uF/160v electrolytic capacitors - I can see the possibility that without this capacitor, negative feedback to the screen grid of the EF86 would reduce gain. See p.28 of the manual.
This one is a real puzzler! At least we now know for sure that it is inside the Eros.
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Thanks to all for the suggestions. I'll get the scope on the intermediate nodes tomorrow evening and report back.
I double-checked the 0.030 and 0.010 caps and the 10.5k resistor, all are in the correct location and solder joints appear to be solid.
Since the identical lack of gain is present in both channels, does that tend to point to a common component to both channels (assuming, for now, that it's not a swapped component in both channels)? Is it possible that the 6922 isn't functioning properly? I have no experience at all with the behavior of a misbehaving tube (would that even manifest in both channels?) and have no idea how to test the tube itself, other than visually verifying that the heater glows.
Grid and plate stoppers all appear attached well and measure to the correct resistances, and the 100uF caps are oriented as per the manual.
Would there be any value in trying to bypass the RIAA equalization to identify it or eliminate it from the equation?
Thanks for the help, everyone. I'm glad this is a puzzler (so far), thinking about it is definitely a learning experience!
-David
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Quick update with measurements at the intermediate points mentioned by Paul.
Measurements were approximately the same for both channels. Measured visually, peak-to-peak, with an o'scope. 20mV input sine wave.
Measurements are at 60Hz, 500Hz, 1kHz, 10kHz, 20kHz
T19/T27: 20mV for all frequencies
T4/T12: 600mV, 220mV, 200mV, 50mV, 30mV
T9/T10: 800mV, 300mV, 200mV, 200mV, 150mV
C3/C8: 600mV, 220mV, 200mV, 50mV, 40mV
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I can't make any sense of the values that you measured without digging into the schematic and I won't be able to do that before Saturday. I hope you get help from either one of the Paul's much sooner. Something is fishy, but what? I bet you didn't need me to tell you that.
In response to Grainger: what really matters at this point are the ratios between the voltages. As long as all values are either RMS or peak-to-peak everything is pretty straightforward.
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Is there any value in disconnecting the EF86 plate from the RIAA filter, leaving only the 75kohm load resistor and checking the gain at the plate there? Or will it be too hard to know what the correct gain should be without the RIAA filter in place?
Similarly, is there any way to isolate the 2nd stage 6922 to test it on its own?
Thanks,
David
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Bingo!
The 100uF is not connected. Perhaps it is in the wrong holes? - there are several in a row.
I determined this because there are large AC voltages on the 6922 cathodes (C3/C8), which should be bypassed to ground by that capacitor.
With respect to questions about the tubes, at least in this circuit, the fact that the DC voltages are right indicates that their gain parameters (transconductance and/or mu) are within specs.
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Thanks, Paul! I won't be able to check until late tonight (or early Saturday morning, depending on how many beers the evening brings), but I'm betting you're right about the wrong hole.
Will update...
-David
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Dave,
You are a true Texan. I remember fond nights at Up In Smoke, a BBQ joint North of Fort Worth. Great place, great wings and very fresh Lone Star beer!
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Not sure if my neighbors would consider me a true Texan (I'm originally from Wilmington/Philadelphia), but I've been here long enough to love good BBQ!
Took a quick look at Page 29 in the manual, and although I have the 100uF cap in the right place, I am missing the 1" red wire that connects the cap to breg...on both boards, of course. Essentially, as Paul stated, the cap is unconnected.
Will add this in the morning and report back.
Thanks,
David
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Updated output voltages...All look good relative to each other, particularly considering margin of error on visual scope measurements.
60Hz: 40v
500Hz: 11v
1kHz: 7v
10kHz: 1.7v
20kHz:1v
And of course, the only test that really matters...Put it back in the system and delight ensued. I have a feeling I won't be getting much done this weekend other than listening. And I'm okay with that.
Thank you very much, everybody, for patience and help in finding this mistake.
-David
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Good news!!! Enjoy. Let us know how you like it as it burns in.
Cheers,
Geary
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Two 1" wires, small cause, big effect. I'm glad you found it. Have some fun listening!
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Impressions so far...WOW. This thing sounds amazing. I've been mostly listening to new music that I've been accumulating while building the Eros, so it's hard to do much A:B comparisons, but I did go back to a couple of old favorites and have been stunned at the sound.
Only question that remains is how long can I hold off before ordering the BeePre... :-)
-David